Urban myth or terror warning? How David Blunkett took a tale of a rumoured attack straight to the top.

It is a story that has dropped in to many an e-mail in-box or been passed on by word-of-mouth. And its distribution has picked up in these times of heightened security since the 11 September and 7 July attacks.

The characters in the story and its location vary, but the plot stays mostly the same. It runs as follows:

"My friend's Aunt Sally was in a queue and this Middle Eastern-looking bloke in front of her dropped his wallet. When she gave it back to him, he told her to avoid central London on Saturday because something big might happen. Tell as many people as you can."

Most people sigh, and delete the e-mail.

But when this same tale crossed David Blunkett's path in late 2001, he passed it straight to the top.

An entry in his newly-published diary reveals how he had spoken to an old school friend, who had heard the story involving the return of a wallet to an Arab man and a warning not to be in London on 11 November.

"I immediately registered the significance of this," Blunkett wrote at the time. "The 11th of November is Armistice Day, the one day in the year when all leading politicians from the three parties, the Queen, other members of the Royal family, and the leading personnel of the armed services are in the same place at the same time - a known time, in central London.

"I decided that I should at least tell Tony Blair as it was absolutely clear that nobody had fully thought through the significance.

"We agreed there was no way we could possibly cancel Armistice Day, but we were certainly going to have to take increased precautions."

Then later: "Sunday 11th of November: And we've come through Remembrance Sunday safely. All the worry was for nothing, thank God."

The basis for all that worry? An urban myth that's been doing the rounds for decades in one form or another. When the troubles in Northern Ireland were at their height, the tipster was an Irishman. The location has moved around the UK - to Birmingham, Coventry, Reading. Abroad it has centred on different US cities.

But it dates back at least as far as a myth surrounding the Hiroshima bombings, says Albert Jack, author of That's B*ll*cks: Urban Legends, Conspiracy Theories and Old Wives' Tales.

In that version Americans were apparently warned to get out of the city before the atomic bomb was dropped.

A similar but also false myth followed the Twin Tower attacks, that Muslim workers had been warned to stay home, and was reported in New York on 12 September.

The terror warning tale is similar in nature to the "missing kidney" and "long-dead hitchhiker" urban myths, says Mr Jack.

The first involves a person who goes to a party with friends, only to wake up in a bath of ice minus their kidneys. The second a driver who gives a girl a life home, only to discover when she disappears that she died years earlier in a car crash.

"All these e-mails are the modern-day version of medieval folklore," says Mr Jack.

"Many people tell stories like this to make themselves more popular. If you stand in the pub and tell stories, a lot of them will make you laugh, and so people think it makes them popular."

Urban myths may arise from a snippet or a fact, or from what conspiracy theorists might like to believe. The origin is hard to pin-point once they've started to spread and mutate. But it is when people in power hear and act on them that they gain credence.

How to guard against falling for one? Mr Jack says: "I always say, 'If it's really true, show me the carcass'. These things are so often repeated, it's unimaginative to tell them, so show me the evidence."

"I immediately registered the significance of this," Blunkett wrote at the time. "The 11th of November is Armistice Day, the one day in the year when all leading politicians from the three parties, the Queen, other members of the Royal family, and the leading personnel of the armed services are in the same place at the same time - a known time, in central London.

"I immediately registered the significance of this," Blunkett wrote at the time. "The 11th of November is Armistice Day, the one day in the year when all leading politicians from the three parties, the Queen, other members of the Royal family, and the leading personnel of the armed services are in the same place at the same time - a known time, in central London.

"I immediately registered the significance of this," Blunkett wrote at the time. "The 11th of November is Armistice Day, the one day in the year when all leading politicians from the three parties, the Queen, other members of the Royal family, and the leading personnel of the armed services are in the same place at the same time - a known time, in central London.